Not applicable.
Not applicable.
This invention is in the field of telecommunications, and is more specifically directed to the digital signal processing of frequency multiplexed signals in such telecommunications.
In recent years, the data rates at which communications may be carried out over conventional telephone networks and wiring has greatly increased. These increases are due, in large part, to newly adopted techniques of multiplexing and modulating signals representative of the messages or data being communicated, resulting in greatly improved communication bandwidth. In addition, the carrier frequencis at which such communications are being carried out have also increased in recent years, futher improving the bit rate.
According to one well-known class of multiplexing, digital data are communicated at multiple sub-carrier frequencies, or tones. This class of frequency multiplexing is referred to as Discrete MultiTone (DMT) in wired communication, or alternatively as Orthogonal Frequency Division Multiplexing (OFDM) in wireless communication. In this type of multiplexing, the stream of data symbols are multiplexed into N parallel subchannels, each subchannel being associated with a sub-carrier frequency. After modulation, the sub-carriers are added and transmitted together as an analog signal; at the receiving end, the sub-channels are filtered from one another and the original non-multiplexed data streanm is recovered.
It is important in this type of multiplexing that neighboring sub-carrier frequencies do not interfere with one another. Of course, wide separation of the sub-carrier frequencies would eliminate such interchannel interference (ICI), but at a cost of low spectral density. A well known method of ensuring orthogonality of the sub-carriers, and thus avoiding ICI, is to utilize a rectangular pulse shape as the sub-carrier pulse. According to the theorems of the Fourier Transform, a rectangular pulse in the time-domain transforms into a sin(x)/x frequency-domain spectrum. In the frequency multiplexing case, this spectrum is centered about the sub-carrier frequency ƒ0, and has an argument x=xcfx80NT(ƒxe2x88x920), where ƒ refers to the actual frequency of the communication, N is the number of parallel subchannels being transmitted, and T is the period of communication of discrete information (i.e., the reciprocal of the symbol communication rate). Proper selection of the sub-carrier frequencies to ensure orthogonality follows the relationship:
ƒk=k/NT 
where k is the sub-carrier index (i.e., the xe2x80x9ctonexe2x80x9d in the multitone set). If this relationship is maintained in assigning the sub-carrier frequencies, each sub-carrier will have a center frequency that is located at a zero crossing of the spectrum of other sub-carriers, and as such each sub-carrier will be orthogonal to the other encoded sub-carriers.
FIG. 1 illustrates the frequency spectrum of a sub-channel in OFDM or DMT transmission. In the frequency response plot of FIG. 1, the frequency axis is measured in sub-carrier index values relative to the index of the center frequency. The center frequency (relative index of 0) provides the maximum frequency response, as shown. At relative index values of xc2x11, for example, the frequency response for the sub-channel is at zero. Accordingly, the sub-channel illustrated in FIG. 1 will provide no contribution at the center frequencies of the adjacent sub-channels (relative index of xc2x11); conversely, since the adjacent sub-channels have the same normalized frequency response as shown in FIG. 1, they will provide no contribution to the sub-channel of FIG. 1 (relative index of 0). Furthermore, as shown in FIG. 1, the frequency response is zero at each integer value of relative index. As such, the illustrated sub-channel provides no contribution to any other sub-channel, and conversely no other sub-channel contributes to the signal at the center frequency of the illustrated sub-channel of FIG. 1. The use of a rectangular pulse thus provides orthogonality among the various sub-channels, permitting close spacing of the center frequencies and thus high spectral density.
In order to maintain orthogonality, however, the modulation and demodulation of the signals must be performed at the same precise center frequencies. As evident from FIG. 1, if demodulation is performed at a frequency that is slightly offset from the center frequency, not only will the frequency response for the desired sub-channel be less than optimal, but the demodulated signal will also contain contributions from other sub-channels; these contributions amount to interchannel interference (ICI), and greatly reduce the signal quality of the system. It is therefore important to ensure precision in the demodulation frequencies in DMT/OFDM communication systems.
The precise matching of modulation and demodulation frequencies is made difficult in modem communications by the physical separation of communicating modems from one another, where each of the communicating modems is driven by its own local clock. Conventional DMT/OFDM modems typically use expensive and complicated circuitry to ensure such precise frequency mathcing. FIG. 2 illustrates the construction of the receiver side of conventional DMT modem 10. As shown in FIG. 2, modem 10 receives signals from the telephone network at analog-to-digital converter (A/D) 14. The signals received by modem 10 include, in addition to the communicated messages, a pilot tone generated by the transmitting modem to communicate the frequency at which it carried out the modulation of the message data. The digital output of A/D 14 is processed by time-domain equalization function 20, cyclic prefix removal function 22, Fast Fourier Transform (FFT) function 24, and frequency domain equalization function 26 (such functions typically performed by digital signal processor, or DSP, 12), following which the received communicated signals are applied, in digital form, to the host computer of modem 10. Temporal control of modem 10 is maintained in response to the pilot tone, as recovered from the received communication by FFT function 24, which generates a digital value corresponding to the instantaneous frequency of this detected tone. The frequency of the pilot tone is filtered by digital filter function 28 (also typically within DSP 12), converted into an analog signal by digital-to-analog converter (D/A) 18, and applied to voltage controlled oscillator (VCXO) 16. VCXO 16 responds to the analog signal corresponding to the pilot tone frequency to control A/D 14, such that the time-domain sampling and conversion of the incoming received communication is performed at a frequency that precisely matches that of the transmitting modem (as communicated by way of the pilot tone). A phase-locked loop (not shown) may also be implemented in conventional modem 10, to ensure stable matching of the output of VCXO 16 relative to the incoming signals.
It has been observed, however, that VCXO 16 is typically an expensive function to include in client-side modem systems, Furthermore, fluctuations in the control voltage appluied to VCXO 16 by D/A 18 directly cause frequency jitter at the output of VCXO 16; such fluctuations are common for modems within electtrically noisy environments such as modern personal computers and workstations. As a result, the conventional modem construction, as shown in FIG. 2, includes expensive oscillator circuitry that still does not provide a high degree of precision in its frequency output when implemented in the usual applications.
A relatively new type of current modem communications technology is referred to in the art as digital subscriber line (xe2x80x9cDSLxe2x80x9d). DSL refers generally to a public network technology that delivers relatively high bandwidth over conventional telephone company copper wiring at limited distances. DSL has been further separated into several different categories of technologies, according to a particular expected data transfer rate, the type and length of medium over which data are communicated, and schemes for encoding and decoding the communicated data. According to this technology, data rates between DSL modem may be far greater than current voice modem rates. Indeed, current DSL systems being tested or projected range in rates on the order of 500 Kbps to 18 Mbps or higher. According to certain conventional techniques, such as the protocol referred to as Asymmetric Digital Subscriber Line (ADSL) and which corresponds to ANSI standard T1.413, the data communication rates are asymmetrical. Typically, the higher rate is provided for so-called downstream communications, that is from the telephone network central office to the customer modem, with upstream communication from the customer modem to the central office having a data rate consideraly lower than the downstream rate.
In current-day ADSL systems operating according to DMT modulation, only one of the communicating modems has a master clock; typically, the central office modem generates this master clock signal. The client modem is thus required to recover the master clock signal from the communicated data stream, and use this clock not only to sample and demodulate the received data stream, but also in its transmission of upstream data to the central office modem. According to the ADSL standard, the central office modem will expect a jitter-free upstream, or reverse link, data stream, sampled and modulated according to the master clock signal.
It is therefore an object of the present invention to provide a low cost modem that provides a high degree of precision in the communication of frequency multiplexed communications.
It is a further object of the present invention to provide such a modem and method of operating the same in which operation of the receiving side of the modem may be driven by a low-cost, free run oscillator.
It is a further object of the present invention to provide such a modem and method of operating the same that utilized digital signal processing (DSP) functionality to correct for phase and frequency offset in the demodulation process.
It is a further object of the present invention to provide correction for reverse link transmission in an Asymmetric Digital Subscriber Line (ADSL) communication environment.
Other objects and advantages of the present invention will be apparant to those of ordinary skill in the art having reference to the following specification together with its drawings.
The present invention may be implemented into receive circuitry in modem, of either the wireless or wired type, for receiving frequency multiplexed communications. The sampling and demodulating circuitry is controlled by a numerically controlled oscillator (NCO) deriving a clock based upon the output of a free run crystal oscillator. Estimation of the phase offset and frequency offset of the demodulation controlled by the oscillator, relative to the transmitted signal, is made by the receive circuitry relative to the transmitted pilot tone. Phase rotation is applied to the receiver signal to compensate for the phase offset. A digital filter, such as a Finite Impulse Response (FIR) filter executed by a digital signal processor (DSP), corrects for frequency offset in the demodulated received signal. As a result, the receive modem can be constructed using relatively low cost clock and oscillator circuitry, while maintaining excellent orthogonality among sub-channels.
According to another aspect of the invention, the receive circuitry constructed according to the present invention is implemented in a client modem receiving and transmitting Asynchronous Digital Subscriber Line (ADSL) communications. The client modem implements a pre-emphasizing operation upon the reverse link transmission signal to compensate for the frequency offset of the free run oscillator or NCO relative to the master clock of the central office modem; a pre-transmission phase rotation pre-compensates for the phase rotation, as well. The upstream transmissions thus arrive at the central office modem without frequency or phase offset caused by the oscillator controlling the client modem.